Pendulum Effect: Part 1

Back before I became a consultant in 1962, I was employed by TransCo. My boss had a number of very descriptive expressions for commonly encountered problem conditions with reciprocating compressors. One of them that I have always appreciated over the previous 30 plus years was his term for pendulum action of a large unsupported mass on the end of a vibrating pipe. He called it "a billiard ball on the end of a fly rod". You avid fisherman will appreciate that one.

The most common pendulum configuration, i.e., billiard ball on a fly rod, encountered in plant design is that shown in Figure 1. Here, a heavy valve has been installed on the end of a small diameter nipple which is welded into the side of a flow line. This practice dates back many years and is still incorporated in plant design drawings, which have never been updated in this respect. It is a design which has very little tolerance for vibration. The heavy mass (the valve) overswings from the vibration in the pipe, creating amplified cyclic stresses in the weld between the nipple and the flow line. Under what would ordinarily be considered reasonable vibration conditions, this nipple will eventually fail.